Ink-jet recording is carried out in such a manner that fine droplets of ink are allowed to fly employing various operation principles and are adhered onto a recording sheet such as a sheet of paper to record images and characters. Ink-jet recording exhibits advantages such as relatively high speed, low noise, ease of multicolor production, and the like. Conventional problems of this system regarding clogging of nozzles and maintenance have been solved by improving both aspects of ink and the device, and at present, ink-jet recording has been rapidly applied to a variety of fields such as various printers, facsimile machines, computer terminals, and the like.
Requirements for a recording sheet employed in such ink-jet recording system is that the density of printing dots is high; color tone is bright and clear; ink is rapidly absorbed and when printing dots are superimposed, ink should neither run out nor blot; the diffusion of a printing dot in the lateral direction should not be greater than that needed and the circumference should be smooth and result in no blurring; and the like.
Particularly, at low rates of ink absorption, when recording is carried out by superimposing at least two color inks, on a recording sheet, droplets result in repellence to cause unevenness and in the boundary area of different colors, mutual colors result in blurring. As a result, image quality tends to be markedly degraded. Therefore, it is required that the recording sheet exhibits high ink absorbability.
In order to solve these problems, conventionally, a great number of techniques have been proposed.
For example, Japanese Patent Publication Open to Public Inspection No. 52-53012 describes a recording sheet in which minimally sized paper support is damped with paint employed for surface treatment; Japanese Patent Publication Open to Public Inspection No. 55-5830 describes a recording sheet comprising a support in which an ink absorbing coating layer is provided on the surface of the support; Japanese Patent Publication Open to Public Inspection No. 56-157 describes a recording sheet comprising a covering layer containing non-colloidal silica powder as a pigment; Japanese Patent Publication Open to Public Inspection No. 57-107873 describes a recording sheet comprising an inorganic pigment and an organic pigment in combination; Japanese Patent Publication Open to Public Inspection No. 58-110287 describes a recording sheet which exhibits a void distribution with two peaks; Japanese Patent Publication Open to Public Inspection No. 62-111782 describes a recording sheet composed of an upper porous layer and a lower porous layer; Japanese Patent Publication Open to Public Inspection Nos. 59-68292, 59-123696, 60-18383, etc. describe a recording sheet having amorphous cracking; Japanese Patent Publication Open to Public Inspection Nos. 61-135786, 61-148092, 62-149475, etc. describe a recording sheet having a fine powder layer; Japanese Patent Publication Open to Public Inspection Nos. 63-252779, 1-108083, 2-136279, 3-65376, 3-27976, etc. describe a recording sheet comprising pigments and fine silica particles having specified physical parameters; Japanese Patent Publication Open to Public Inspection Nos. 57-14091, 60-219083, 60-210984, 61-20797, 61-188183, 5-278324, 6-92011, 6-183134, 7-137431, 7-276789, etc. describe a recording sheet containing fine silica particles such as colloidal silica; Japanese Patent Publication Open to Public Inspection Nos. 2-276671, 3-67684, 3-215082, 3-251488, 4-67986, 4-263983, 5-16517, etc. describe a recording sheet containing fine hydrated alumina particles, and the like.
Of these, a recording sheet provided with a void layer composed of fine organic particles having an average diameter of not more than 100 nm and a hydrophilic binder as an ink absorbing layer is preferred, which employs a technique for achieving high ink absorbability and glossiness at the same time.
Further, with such a void layer, a large content of fine inorganic particles with respect to a hydrophilic binder is needed for the formation of voids, and the resulting void layer becomes rigid due to markedly high rigidity. Such a rigid layer is required to obtain high ink absorbability. However, it has been found that during the coating and drying processes of the void layer, cracking readily occurs in the presence of tiny foreign matter, due to low flexibility of the layer and problems are caused in a case of the dry void layer having a thickness of not less than 25 .mu.m, which is specifically required for ink-jet recording.
It has been found that when a recording sheet having such a void layer is produced, relatively coarse particles such as coarse inorganic particles themselves, coagulum of fine inorganic particles, coagulum of other additives tend to be formed due to various causes during preparing the coating composition, and these coarse particles cause cracking during coating and drying onto a support.